In the chemiluminescence analyzer for measuring exhaust gas from automobiles, dilution is carried out to lower the concentration of CO.sub.2 or H.sub.2 O in the sample gas in a reactor to reduce quenching action caused by these gases.
For the diluting gas, O.sub.2 gas, which is the source gas of the ozone generator, is used, and a mixing point a of the diluting gas is installed, for example, at the downstream side of a NO.sub.x converter c in a sample gas line b, as shown in FIG. 2. In FIG. 2, letter d denotes a sample gas inlet; e, a filter; f, an atmospheric chemiluminescence analyzer; g, a deozonizer (ozone decomposer); h, a flow sensor; i, a sample gas outlet; j, an O.sub.2 gas inlet; k, a diluting gas line; and l, an ozonizer (ozone generator).
As described above, if the diluting gas mixing point a is installed at a location downstream of the NO.sub.x converter c, by-products generated by a same specific component (e.g., high boiling-point hydrocarbons) in the exhaust gas may deposit in the sample gas line b between the mixing point a and the NO.sub.x converter c and clog the piping after the components pass the NO.sub.x converter c.